When sewing having sewing machines with a thread cut-off device, wherein the needle thread or upper thread, respectively, and the hook thread or lower thread, respectively, are automatically cut off at the end of the sewing action, there is the basic problem that the thread ends hanging down from the workpiece must be severed, i.e. the workpiece must be trimmed. The same applies to the thread ends at the starting point of the seam.
In order to solve this problem it is known from U.S. Pat. No. 2,707,927 for household sewing machines to secure clamping and cutting plates pivotably to the fabric presser foot and to provide a pivotably supported knife below the stitch plate, i.e. directly below the stitch hole. These two knifes are actuated by means of two electro-magnets to be actuated simultaneously. Thus the needle thread is cut off on the upper side of the workpiece and the lower thread is cut off directly below the stitch hole. With this thread cut-off device the cutting off of threads takes place in such a way that only infinitely short thread ends remain which need no more trimming.
From U.S. Pat. No. 4,674,425 a thread trimming head is known with a heating wire to sever a thread chain thermically and with a suction tube to guide the thread chain to the heating wire. The heating wire is arranged laterally outside the suction tube to protect it from extreme cooling through the air current. This thread trimming head is arranged above the stitch forming place and serves in particular to sever the thread chain of double-chain-stitch sewing machines.
U.S. Pat. No. 3,159,124 arranges a suction tube adjacent to the stitch forming place to suck in the thread or the thread chain, respectively. A heating wire in this suction tube melts the sucked-in thread thermically.
It is further known to trim a workpiece, which has been sewn and taken out of the sewing machine, by means of trimming devices of the above-mentioned kind.